KR100514297B1 - Apparatus and method for compensating residual frequency offset of wireless local area network system - Google Patents

Abstract

The present invention relates to an apparatus and method for compensating for a residual frequency offset in a wireless LAN system.

In the present invention, before compensating for the residual frequency offset, the channel compensation is first performed on the first OFDM symbol received from the transmitting end of the WLAN system, and then the sampling frequency is estimated according to the phase error estimation value of the first OFDM symbol with channel compensation. Calculate the offset. In this case, the present invention also calculates the bias frequency offset so that the bias frequency offset can also be compensated for in the sampling frequency offset compensation of the second OFDM symbol. This not only allows the residual frequency offset to be gradually reduced to improve the performance of the wireless LAN system, but also performs channel compensation first and then performs residual frequency offset compensation later, thereby enabling stable system implementation.

Description

Residual Frequency Offset Compensation Device in Wireless LAN System and Its Method {APPARATUS AND METHOD FOR COMPENSATING RESIDUAL FREQUENCY OFFSET OF WIRELESS LOCAL AREA NETWORK SYSTEM}

The present invention relates to an apparatus and method for compensating for a residual frequency offset, and more particularly, to an apparatus and method for compensating for a residual frequency offset including a bias frequency offset in a wireless LAN system. will be.

In general, an orthogonal frequency division multiplexing (OFDM) system (transmitter) converts a high-speed data signal into a low-speed data signal and then has a predetermined number of subcarriers (subcarriers) having orthogonality to each other. The carrier modulates the data signals on the carriers and sends them to the receiver.

However, in such an OFDM scheme, when a frequency offset between transmission and reception ends occurs, orthogonality between subcarriers is reduced, which causes channel interference called ICI (Inter Channel Interference), thereby changing the magnitude and phase of a signal to be transmitted and received. Let's do it.

Therefore, in the high-speed wireless LAN system using the OFDM scheme, a preamble section consisting of 10 short training signal sequences and two long training signal sequences is generally transmitted before data is transmitted. These two types of training signals are used to compensate for phase distortion. In this case, a guard interval (GI) is transmitted between the preamble intervals to prevent the occurrence of an InterSIymbol Interference (ISI).

Then, when the WLAN system (transmitter) transmits the corresponding data to the receiver, it is loaded on the subcarrier via FFT (Fast Fourier Transform, hereinafter referred to as 'FFT'), where the receiver uses a long training signal sequence. The received data is decoded through estimating and compensating for the characteristics of the transport channel.

However, the above-described method does not compensate the phase error accurately in the process of accurately estimating and compensating the degree of phase distortion, so that residual phase distortion occurs, which causes residual frequency offset of the subcarrier. In addition, the sampling frequency offset may occur due to an incorrect sampling frequency.

Therefore, when the above-described offsets are generated, an incorrect decoding is performed due to the residual offset even if the channel is compensated.

In detail, the conventional receiver (residual frequency offset compensation device) first performs channel compensation on the first OFDM symbol (a data unit carried on a predetermined number of subcarriers) received from the transmitter, and then performs channel compensation. First phase error information is estimated using pilot information of one OFDM symbol.

Thereafter, the receiving end estimates and compensates the subcarrier frequency offset and the sampling frequency offset using the first phase error information previously estimated for the second OFDM symbol to be received. After receiving the second phase error information by using the second OFDM symbol, the receiving end compensates the third OFDM symbol by summing it with the first phase error information.

A display example of such an operation process is shown in FIG. 1.

1 is a graph conceptually illustrating a residual frequency offset compensation result according to the prior art.

As shown in FIG. 1, the residual frequency offset compensation result according to the related art shows that there is a residual frequency offset Δ ₀ having a predetermined magnitude from the first OFDM symbol to the second and third OFDM symbols. have.

This is because the phase error and the carrier frequency offset of the current OFDM symbol only compensate for the residual frequency offset of the OFDM symbol to be received next time, but not the frequency offset compensation for itself.

That is, the first OFDM symbol is output as it is without phase error compensation, that is, carrier frequency offset compensation, and the second OFDM symbol is output with only the first carrier frequency offset compensated, and the third OFDM symbol is first The accumulated value of the carrier frequency offset and the second frequency offset is compensated for and output. This results in the overall bias frequency offset remaining.

As a technique related to such an offset compensation method, the patent holder is Samsung Electronics Co., Ltd. [Patent Name: OFDM reception system and method for estimating an efficient symbol timing offset, registration number: 10-2001-32495] By estimating the symbol timing offset by accumulating the real and imaginary numbers of the phase rotation difference information, respectively, the symbol timing offset insensitive to the influence of the channel can be estimated even with a small amount of calculation.

However, the above technique is difficult to implement a stable OFDM reception system by first estimating a symbol timing offset and then performing channel compensation thereafter.

The technical problem to be solved by the present invention is to solve this problem, by calculating the bias frequency offset in accordance with the phase error estimation value of the OFDM symbol with the channel compensation to compensate with the carrier frequency offset up to the bias frequency offset of the OFDM symbol, It is an object of the present invention to provide an apparatus and method for compensating residual frequency offset in a wireless LAN system that can gradually reduce frequency offset to improve system performance.

An apparatus for compensating residual frequency offset in a wireless LAN system according to an aspect of the present invention for achieving the above object is an apparatus for compensating for the residual frequency offset of an orthogonal frequency division multiplexing (OFDM) symbol received from a transmitting end of a wireless LAN system. A channel compensator for estimating a channel state using a long training signal sequence received from the transmitter and compensating for an OFDM symbol received from the transmitter using the estimated channel state information; An error estimator for extracting at least one pilot of the compensated OFDM symbol and then comparing the extracted pilot with previously known pilot information to estimate a phase error of the OFDM symbol; A carrier frequency offset calculator configured to calculate an accumulated carrier frequency offset of the OFDM symbol received from the transmitter according to the estimated phase error; A bias frequency offset calculator configured to calculate a bias frequency offset of the compensated OFDM symbol by adding a parameter having a predetermined size to a carrier frequency offset of a previous OFDM symbol received before the compensated OFDM symbol; And a carrier frequency offset compensator configured to add the calculated bias and the accumulated carrier frequency offset to compensate and output a carrier frequency offset of the next OFDM symbol received after the OFDM symbol.

And an FFT unit for fast Fourier transforming the OFDM symbols received from the transmitter and outputting the OFDM symbols to the channel compensation unit; A sampling frequency offset calculator for calculating a cumulative sampling frequency offset of an OFDM symbol received from the transmitter according to a phase error estimated by the channel estimator; An FFT window start position adjusting unit for adjusting the start position of sampling by informing the FFT unit if the sampling position of the OFDM symbol to be sampled is out of a reference value according to the calculated cumulative sampling frequency offset; And a sampling frequency offset compensator for compensating for and outputting a sampling frequency offset of a next OFDM symbol that is output after the sampling start position is adjusted.

In addition, a method of compensating for the residual frequency offset of an Orthogonal Frequency Division Multiplexing (OFDM) symbol received from a transmitting end of a wireless LAN system according to an aspect of the present invention, a) a channel state using a long training signal sequence received from the transmitting end Estimating, and compensating for an OFDM symbol received from the transmitter using the estimated channel state information; b) extracting at least one pilot of the compensated OFDM symbol, and then comparing the extracted pilot with previously known pilot information to estimate a phase error of the OFDM symbol; c) calculating the accumulated carrier frequency offset of the OFDM symbol received from the transmitting end according to the estimated phase error of the OFDM symbol; d) calculating a bias frequency offset of the OFDM symbol by adding a parameter of a predetermined size to a carrier frequency offset of a previous OFDM symbol received before the OFDM symbol; And e) summing the calculated bias and cumulative carrier frequency offset and compensating the carrier frequency offset of the next OFDM symbol received after the OFDM symbol with the summed frequency offset.

And f) calculating a cumulative sampling frequency offset of the OFDM symbol received from the transmitter according to the estimated phase error of the OFDM symbol; g) if the sampling position of an OFDM symbol to be sampled is out of a reference value according to the calculated cumulative sampling frequency offset, adjusting a start position of sampling by the deviation value; And h) compensating for and outputting the sampling frequency offset of the next OFDM symbol which is adjusted and outputted at the start position of the sampling.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention. Like parts are designated by like reference numerals throughout the specification.

2 is a view showing in detail the residual frequency offset compensation device according to an embodiment of the present invention.

As shown in FIG. 2, the residual frequency offset compensator 100 according to an exemplary embodiment of the present invention includes an FFT unit 110, a channel compensator 120, an error estimator (dotted line display), and a carrier frequency offset calculator ( 150, the bias frequency offset calculator 160, the adder 170, the carrier frequency offset compensator 180, the sampling frequency offset calculator 190, the FFT window start position adjuster 200, and the sampling frequency offset compensator And 210. In this case, the error estimator includes a pilot extractor 130 and a phase error estimator 140.

In detail, first, the FFT unit 110 converts an OFMD symbol (also known as an OFDM symbol currently received as a first OFDM symbol) received from a transmitting end of a wireless LAN system to a FFT (Fast Fourier Transform). Conversion process.

In this case, the OFDM symbol received from the transmitting end of the WLAN system includes 48 data and 4 pilots carried on 52 subcarriers. Such a configuration is not limited to the present invention, and in some cases, may include more or less data numbers.

Next, the channel compensator 120 estimates the channel state using the long training signal sequence received before the first OFDM symbol received from the FFT unit 110 and then uses the estimated channel state information to determine the first OFDM. Compensate the symbol.

As described above, in the embodiment of the present invention, in order to stabilize the WLAN system, channel compensation is first performed before compensating the residual frequency offset, and then the residual frequency offset is compensated based on the OFDM symbol on which the channel compensation is performed.

The pilot extractor 130 extracts pilots (about 4 as described above) of the first OFDM symbol received from the channel compensator 120, that is, the OFDM symbol for which channel compensation is performed.

The phase error estimator 140 estimates the difference error of the first OFDM symbol by comparing the extracted pilot information with pilot information already known at the receiving end. At this time, the phase error estimating unit 140 estimates the phase error, that is, the phase distortion, in symbol units.

The carrier frequency offset calculator 150 calculates a carrier frequency offset of the first OFDM symbol according to the estimated phase error. The carrier frequency offset calculator 150 also calculates a carrier frequency offset in OFDM symbol units.

The bias frequency offset calculator 160 calculates a bias offset generated due to the carrier frequency offset of the first OFDM symbol according to the estimated phase error, which is appropriate between 0.6 and 0.9 times the carrier frequency offset obtained from the previous symbol. The sum of the values. However, this is not limited to the present invention, and in some cases, additional values may be added or subtracted.

Next, the adder 170 sums and outputs the calculated carrier frequency offset and bias frequency offset of the first OFDM symbol.

As such, the residual frequency offset compensation apparatus 100 according to an embodiment of the present invention compensates for the carrier frequency offset of the second OFDM symbol by using the first OFDM symbol with channel compensation. By further calculating, summing and outputting the bias frequency offset, which was an inevitable residual frequency offset, the residual frequency offset of the second OFDM symbol can be compensated for.

In addition, since the residual frequency offset compensator 100 is further configured to accumulate and output the newly calculated bias frequency offset and the previously calculated bias frequency offset, the third OFDM symbol may also be output as the second OFDM symbol. Larger residual frequency offsets are compensated for than OFDM symbols.

Next, the carrier frequency offset compensator 180 compensates for the carrier frequency offset of the second OFDM symbol according to the sum of the carrier and the bias frequency offset received from the adder 170.

The sampling frequency offset calculator 190 calculates a sampling frequency offset of the first OFDM symbol according to the previously estimated phase error.

The FFT window start position adjuster 200 determines whether the position of the OFDM symbol to be sampled is out of the range of the frequency offset of a single sample according to the calculated sampling frequency offset. As a result of the check, if the reference value is out of the FFT window start position adjusting unit 200 notifies this to the FFT unit 110 to adjust the sampling start position. That is, the FFT unit 110 adjusts the start position of the window according to the position adjustment instruction of the FFT window start position adjuster 200.

The sampling frequency offset compensator 210 compensates and outputs the sampling frequency offset of the second OFDM symbol according to the calculated sampling frequency offset. The second OFMD symbol thus output is decoded through a decoder (not shown).

Here, some of the above-described functional units will be described with reference to the accompanying drawings.

FIG. 2 is a diagram illustrating in detail the configuration of the channel compensator 120 illustrated in FIG. 1.

As shown in FIG. 2, the channel compensator 120 includes a delay unit 121, an adder 122, a divider 123, a multiplier 124, a memory 125, and a compensator 126. Include.

First, the delay unit 121 stores the long training signal sequence received from the FFT unit 110 until the calculation of the channel estimation in the adder 122 is completed.

That is, the adder 122 calculates the sum of the squares of the I channel and the Q channel for the received long training signal sequence.

The divider 123 divides the received long training signal sequence by the sum of squares of the obtained channels, and the multiplier 124 multiplies the channel information obtained by the divider 123 with the already known long training signal sequence information.

The memory unit 125 stores the result value through this process, and the compensation unit 126 performs channel compensation through the result value when the first OFMD symbol comes in after a long training signal sequence.

Next, FIG. 4 is a diagram showing in detail the configuration of the carrier frequency offset calculator shown in FIG.

As shown in FIG. 4, the carrier frequency offset calculator 150 includes a first accumulator 151, an average unit 152, a first multiplier 153, a second multiplier 154, and a second accumulator. 155 and the adder 156.

The first accumulator 151 accumulates the estimated phase error received from the phase error estimator 140, and the average unit 152 obtains an average value of the accumulated phase errors.

The first multiplier 153 multiplies the average value obtained by the average unit 152 by a first parameter having a predetermined size, while the second multiplier 154 multiplies the average value obtained by the average unit 152 with the first parameter. And multiply a second parameter having a different size, and the second accumulator 155 accumulates the multiplied value in symbol units.

The adder 156 adds the result values received from the first multiplier 153 and the second multiplier 154 to estimate the carrier frequency offset.

5 is a diagram illustrating in detail the configuration of the sampling frequency offset calculator shown in FIG.

As shown in FIG. 5, the sampling frequency offset calculator 190 includes a first multiplier 191, a first accumulator 192, a divider 193, a second multiplier 194, and a third multiplier. 195, a second accumulator 196 and an adder 197.

The first multiplier 191 multiplies the phase error received from the phase error estimator 140 by the position information of the pilot, and the first accumulator 192 accumulates the result value obtained by multiplying the position information.

The divider 193 divides the accumulated result by the sum of the squares of each of the I and Q channels, and the second multiplier 194 multiplies the divided result by the first parameter of a predetermined size, while the third multiplier (195) multiplies the divided result by a second parameter having a different size from the first parameter.

The second accumulator 196 accumulates the result value multiplied by the second parameter in symbol units, and the adder 197 adds the accumulated result value and the result value received from the second multiplier 194 to obtain a sampling frequency. Estimate the offset.

As such, the apparatus 100 for compensating the residual frequency offset in the WLAN system according to an exemplary embodiment of the present invention may stabilize the system by compensating for the residual frequency offset based on the OFDM symbol on which the channel compensation is performed.

In addition, the residual frequency offset compensator 100 calculates a carrier frequency offset of an OFDM symbol and subsequently adjusts a phase error of an OFDM symbol to be received, and calculates a phase error by calculating a bias frequency offset according to the estimated phase error of the OFDM symbol. The adjustments should be made together. Through this, not only the residual frequency offset generated for each OFDM symbol can be reduced more and more, which can prevent the performance degradation of the system.

Then, the operation process of the residual frequency offset compensation device constituting such a configuration will be described with reference to the accompanying drawings.

6 is a flowchart sequentially illustrating an operation process of a residual frequency offset compensation apparatus according to an exemplary embodiment of the present invention.

As illustrated in FIG. 6, the FFT unit 110 of the residual frequency offset compensating apparatus 100 performs FFT conversion on an OFMD symbol (aka, a first OFDM symbol) received from a transmitting end of a wireless LAN system (S610).

Thereafter, the channel compensator 120 estimates the channel state by using the long training signal sequence received before the first OFDM symbol received from the FFT unit 110 and then uses the estimated channel state information. Compensating the first OFDM symbol (S620).

As described above, in the embodiment of the present invention, in order to stabilize the WLAN system, channel compensation is first performed before compensating the residual frequency offset, and then the residual frequency offset is compensated based on the OFDM symbol on which the channel compensation is performed.

Thereafter, the pilot extractor 130 extracts a pilot of the first OFDM symbol received from the channel compensator 120, that is, the OFDM symbol with channel compensation, and the phase error estimator 140 extracts the pilot information and the receiving end. By comparing the pilot information already known in the estimation of the difference error of the first OFDM symbol (S630). At this time, the phase error estimating unit 140 estimates the phase error, that is, the phase distortion, in symbol units.

Thereafter, the bias frequency offset calculator 160 calculates a bias frequency offset caused by the carrier frequency offset of the first OFDM symbol according to the estimated phase error (S641).

At this time, if the carrier frequency offset of the previously received OFDM symbol is 1, the bias frequency offset calculator 160 calculates the bias frequency offset of the received OFDM symbol to be 1.6 to 1.9.

The bias frequency offset calculator 160 calculates a bias frequency offset due to accumulation. For example, when the bias frequency offset of the third OFDMA symbol is calculated according to the estimated phase error of the third OFDM symbol, the bias frequency offset calculator 160 may calculate the bias frequency offset. The bias frequency offset accumulated up to the bias frequency offset of the second OFDM symbol is calculated.

This is to gradually reduce the residual frequency offset of the OFDM symbol by compensating for the residual frequency offset of the sequentially incoming OFDM symbol to be further compensated up to the bias frequency offset compared to the phase error. This improves the performance of the system.

A display example of such a residual frequency offset compensation result is shown in FIG. 7.

7 is a graph conceptually illustrating a residual frequency offset compensation result according to an exemplary embodiment of the present invention.

As shown in Figure 7, the residual frequency offset compensating apparatus 100 according to an embodiment of the present invention, the bias frequency offset of a first predetermined size (△ _O) for the OFDM symbols not the carrier and sampling frequency offset is compensated left On the other hand, it can be seen that the residual frequency offset (Δ _1- > Δ ₂ ) gradually decreased toward second and third OFDM symbols received thereafter. This means improving the performance of the WLAN system.

Meanwhile, the carrier frequency offset calculator 150 calculates a carrier frequency offset of the first OFDM symbol according to the estimated phase error (S642).

At this time, the carrier frequency offset calculation unit 150 also calculates the carrier frequency offset by accumulation, for example, when calculating the carrier frequency offset of the third OFDM symbol according to the phase error estimate value of the third OFDM symbol, The carrier frequency offset accumulated up to the carrier frequency offset of the second OFDM symbol is calculated. This is to compensate for the residual frequency offset of the incoming OFDM symbol by increasing linearly.

Subsequently, the adder 170 adds the calculated carrier frequency offset and the bias frequency offset of the first OFDM symbol (S643), and the carrier frequency offset compensator 180 adds the frequency offsets received from the adder 170. According to the result, the carrier frequency offset of the second OFDM symbol is compensated (S644).

Meanwhile, the sampling frequency offset calculator 190 calculates the sampling frequency offset of the first OFDM symbol according to the phase error of the first OFDM symbol previously estimated (S651).

Thereafter, the FFT window start position adjusting unit 200 checks whether the position of the sampled symbol is out of the range of the frequency offset of the single sample according to the calculated sampling frequency offset (S652).

As a result, if the reference value is out of the reference value, the FFT window start position adjusting unit 200 notifies the FFT unit 110 to adjust the position of the window of the FFT (S653), and then assigns a subcarrier at the sampling frequency offset. The frequency is subtracted (S654).

Thereafter, the sampling frequency offset compensator 210 compensates and outputs the sampling frequency offset of the second OFDM symbol whose carrier frequency offset is compensated (S660).

As described above, the apparatus and method for compensating the residual frequency offset in a WLAN system calculate a carrier frequency offset based on a phase error estimation value of a current OFDM symbol on which channel compensation has been performed, and then enter an OFDM symbol later according to the calculated carrier frequency offset. To compensate for the carrier frequency offset.

In this case, the residual frequency offset compensator also calculates a bias frequency offset based on the phase error estimation value, thereby compensating for the bias frequency offset of an incoming OFDM symbol.

However, at this time, the bias frequency offset calculated by the residual frequency offset compensation device is a cumulative amount of the bias frequency offsets of at least one or more OFDM symbols previously received. Reduce it.

The drawings and detailed description of the invention are exemplary only, and are used for the purpose of illustrating the invention only, and are not intended to be limiting or to limit the scope of the invention described in the claims. Therefore, those skilled in the art will understand that various modifications and equivalent other embodiments are possible from this. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

Residual frequency offset compensation apparatus and method in a wireless LAN system according to the present invention can achieve a stable system implementation by compensating the residual frequency offset based on the OFDM symbol is the channel compensation.

In addition, the present invention has the effect of improving the performance of the system by gradually reducing the bias frequency offset after the residual frequency offset compensation by calculating the bias frequency offset based on the phase error estimate and compensating the bias frequency offset of the OFDM symbol. .

1 is a graph conceptually illustrating a residual frequency offset compensation result according to the prior art.

2 is a view showing in detail the residual frequency offset compensation device according to an embodiment of the present invention.

3 is a diagram illustrating in detail the configuration of the channel equalizer shown in FIG. 2.

FIG. 4 is a diagram illustrating in detail the configuration of the carrier frequency offset calculator shown in FIG. 2.

FIG. 5 is a diagram illustrating in detail the configuration of the sampling frequency offset calculator shown in FIG. 2.

6 is a flowchart sequentially illustrating an operation process of a residual frequency offset compensation apparatus according to an exemplary embodiment of the present invention.

7 is a graph conceptually illustrating a residual frequency offset compensation result according to an exemplary embodiment of the present invention.

Claims (12)

An apparatus for compensating for a residual frequency offset of an orthogonal frequency division multiplexing (OFDM) symbol received from a transmitting end of a wireless LAN system, A channel compensator for estimating a channel state using a long training signal sequence received from the transmitter, and compensating for an OFDM symbol received from the transmitter using the estimated channel state information; An error estimator for extracting at least one pilot of the compensated OFDM symbol and then comparing the extracted pilot with previously known pilot information to estimate a phase error of the OFDM symbol; A carrier frequency offset calculator configured to calculate an accumulated carrier frequency offset of the OFDM symbol received from the transmitter according to the estimated phase error; A bias frequency offset calculator configured to calculate a bias frequency offset of the compensated OFDM symbol by adding a parameter having a predetermined size to a carrier frequency offset of a previous OFDM symbol received before the compensated OFDM symbol; And A carrier frequency offset compensator configured to add the calculated bias and the accumulated carrier frequency offset to compensate and output a carrier frequency offset of the next OFDM symbol received after the OFDM symbol Residual frequency offset compensation device in a WLAN system comprising a. According to claim 1, An FFT unit for fast Fourier transforming the OFDM symbols received from the transmitter and outputting the OFDM symbols to the channel compensation unit; A sampling frequency offset calculator for calculating a cumulative sampling frequency offset of an OFDM symbol received from the transmitter according to a phase error estimated by the channel estimator; An FFT window start position adjusting unit for adjusting the start position of sampling by informing the FFT unit if the sampling position of the OFDM symbol to be sampled is out of a reference value according to the calculated cumulative sampling frequency offset; And A sampling frequency offset compensator for compensating and outputting a sampling frequency offset of a next OFDM symbol output after the sampling start position is adjusted Residual frequency offset compensation device in the WLAN system further comprising. The method of claim 2, The channel compensator, A delay unit for delaying the long training signal sequence received from the FFT unit; An adder configured to add the squares of the I channel and the Q channel, respectively, the position information of the received long training signal sequence; A division unit for dividing the delayed long training signal sequence by the sum result; A multiplier for multiplying the already known long training signal sequence by the channel information which is the divided result; A memory unit for accumulating and storing the multiplied result in symbol units; And Compensator for compensating the stored cumulative value to the OFDM symbol received after the long training signal sequence Residual frequency offset compensation device in a WLAN system comprising a. The method of claim 2, The carrier frequency offset calculator, A first accumulator for accumulating the phase errors of the OFDM symbols received from the error estimator in symbol units; An average unit for obtaining an average value of the accumulated phase errors; A first multiplier for multiplying the obtained average value by a first parameter having a predetermined size; A second multiplier for multiplying the obtained average value by a second parameter having a different size from the first parameter; A second accumulator for accumulating the result values of the second multiplier in symbol units; And An adder configured to calculate a carrier frequency offset of the OFDM symbol by summing result values received from the first multiplier and the second multiplier Residual frequency offset compensation device in a WLAN system comprising a. The method of claim 2, The sampling frequency offset calculation unit, A first multiplier for multiplying a phase error of an OFDM symbol received from the phase error estimator with position information of a corresponding pilot; A first accumulator for accumulating the multiplied result in symbol units; A division unit for dividing the accumulated result value by the sum of squares of each of I and Q channels as position information; A second multiplier for multiplying the divided result by a first parameter having a predetermined size; A third multiplier for multiplying the division result by a second parameter having a different size from the first parameter; A second accumulator for accumulating the result values of the second multiplier in symbol units; And An adder configured to calculate a sampling frequency offset of the OFDM symbol by summing result values received from the second multiplier and a third multiplier Residual frequency offset compensation device in a WLAN system comprising a. The method of claim 2, The bias frequency offset calculator, And calculating a bias frequency offset of the OFDM symbol by adding a specific value to the carrier frequency offset of the previous OFDM symbol. A method for compensating for the residual frequency offset of an Orthogonal Frequency Division Multiplexing (OFDM) symbol received from a transmitting end of a wireless LAN system, a) estimating a channel state using a long training signal sequence received from the transmitter, and then compensating for an OFDM symbol received from the transmitter using the estimated channel state information; b) extracting at least one pilot of the compensated OFDM symbol, and then comparing the extracted pilot with previously known pilot information to estimate a phase error of the OFDM symbol; c) calculating the accumulated carrier frequency offset of the OFDM symbol received from the transmitting end according to the estimated phase error of the OFDM symbol; d) calculating a bias frequency offset of the OFDM symbol by adding a parameter of a predetermined size to a carrier frequency offset of a previous OFDM symbol received before the OFDM symbol; And e) after summing the calculated bias and cumulative carrier frequency offset, compensating the carrier frequency offset of the next OFDM symbol received after the OFDM symbol with the summed frequency offset; Residual frequency offset compensation method in a WLAN system comprising a. The method of claim 7, wherein f) calculating a cumulative sampling frequency offset of the OFDM symbol received from the transmitting end according to the estimated phase error of the OFDM symbol; g) if the sampling position of an OFDM symbol to be sampled is out of a reference value according to the calculated cumulative sampling frequency offset, adjusting a start position of sampling by the deviation value; And h) compensating and outputting a sampling frequency offset of a next OFDM symbol which is adjusted and outputted at a start position of the sampling; Residual frequency offset compensation method in a WLAN system further comprising. The method of claim 7, wherein i) fast Fourier transforming and outputting the OFDM symbol received from the transmitter Residual frequency offset compensation method in a WLAN system further comprising. The method of claim 8, Step a) is Delaying the receiving long training signal sequence; Squarely summing each of the I and Q channels, which are position information of the long training signal sequence, while delaying the long training signal sequence; Dividing the delayed long training signal sequence by the sum result; Multiplying the already known long training signal sequence by the channel information which is the divided result; Accumulating the multiplied result in symbol units; And Compensating the accumulated result value for an OFDM symbol received after the long training signal sequence Residual frequency offset compensation method in a WLAN system comprising a. The method of claim 8, Step c) is Accumulating the phase error of the OFDM symbol estimated in step b) in symbol units; Obtaining a phase error average value of the accumulated symbol unit; Multiplying the obtained average value by a first parameter having a predetermined size; Multiplying the obtained average value by a second parameter having a different size from the first parameter; Accumulating a result value multiplied by the second parameter in symbol units; And Calculating a cumulative carrier frequency offset of the OFDM symbol by summing up the accumulated result value and a result value multiplied by the first parameter; Residual frequency offset compensation method in a WLAN system comprising a. The method of claim 8, Step f), Multiplying the position error of the pilot by the phase error of the OFDM symbol estimated in step b); Accumulating the multiplied result in symbol units; Dividing the accumulated result by the sum of squares of each of I and Q channels, which are position information of each OFDM symbol; Multiplying the divided result by a first parameter having a predetermined size; Multiplying the result of division by a second parameter having a different size from the first parameter; Accumulating a result value multiplied by the second parameter in symbol units; And Calculating a sampling frequency offset of the OFDM symbol by summing the accumulated result value and a result value multiplied by the first parameter; Residual frequency offset compensation method in a WLAN system comprising a.